This invention relates generally to the art of magneto-electric printing, and more particularly to a method of increasing the image exposure and developing sensitivity in a magneto-electric printing system employing magnetic toner particles.
By the term "magneto-electric printing system," as used herein, is meant a printing system wherein an image is produced on a recording element with magnetic toner particles that are acted upon by both magnetic and electrostatic forces.
It has been proposed to reproduce an image on a recording element by a magneto-electric printing system employing magnetic toner particles to define the image. In this printing system, the recording element, comprising a photoconductive layer on a light-transmitting substrate, is uniformly electrostatically charged and uniformly toned with magnetic toner particles. The toned and unexposed recording element is then disposed, in darkness, in a sub-threshold magnetic field (a field of magnetic strength insufficient to overcome the electrostatic forces attracting the magnetic toner particles to the recording element in darkness), and then exposed with the image to be reproduced. The resulting increase in the conductivity of the photoconductive layer in the exposed (light-struck) portions reduces the electrostatic attraction between the magnetic toner particles and the photoconductive layer thereat so that the affected magnetic toner particles are removed from the surface of the recording element by the magnetic field. The magnetic toner particles that remain on the recording element define the desired image.
While the aforementioned magneto-electric printing system is satisfactory for many applications, it has been observed, in some cases, that not enough of the magnetic toner particles are removed from those portions of the recording element that have been exposed during the exposure operation. This is especially true when the time of the image exposure is of insufficient duration and/or the intensity of the light image is of insufficient magnitude to increase the conductivity of the photoconductive layer sufficiently to reduce the electrostatic forces thereon. Under these conditions, the magnetic forces on the magnetic toner particles cannot overcome the electrostatic forces thereon to remove them.